1. Field of the Invention
This invention is related to interfacing an ion source generating ions at ambient pressure conditions to a mass spectrometer. More specifically, it is related to interfacing an atmospheric pressure ion source to a portable mass spectrometer.
2. Description of the Related Art
Most of commercial mass spectrometers (MS) are made for lab use and usually are equipped with various ionization sources to analyze different chemical compounds. Atmospheric pressure (AP) ionization sources are of special importance as these sources are efficient and interchangeable to cover wide range of chemicals using the same mass analyzer. Typical examples of the atmospheric ionization methods are: electrospray ionization (ESI), atmospheric pressure chemical ionization (APCI), atmospheric pressure matrix-assisted laser desorption/ionization (AP-MALDI), atmospheric pressure photoionization (APPI), direct analysis in real time (DART) available from IonSense, Inc., desorption ESI (DESI), secondary ESI (s-ESI) and any combination thereof. As mass analyzers operate in vacuum, a common feature of all mass spectrometers utilizing AP ionization sources is an atmospheric pressure interface (API) which serves the function of ion delivery from the atmospheric (ambient) pressure conditions existing in the atmospheric pressure ion source to the vacuum of the mass spectrometer with minimal ion loss for further MS analysis. The API design is important for mass spectrometer performance as it determines its sensitivity.
Modern mass spectrometers utilize complicated API designs to achieve the highest sensitivity. Typically, the API design includes a gas inlet port (typically, a thin capillary or small orifice) and several differentially pumped vacuum sections separated by conductance limiting orifices so the ions move from one section to the other through the conductance orifices while the gas is pumped out separately from each section to get the best vacuum in the last vacuum section. Ion guides used for guiding ions through the differentially pumped vacuum sections can operate using different physical principles based on static (DC) and radio-frequency (RF) electric fields, magnetic field, and/or gas dynamic methods.
An API design is a tradeoff between mutually contradicting requirements for gas throughput, ion guidance/focusing, and gas loads on the vacuum pumps of the differentially pumped sections. It is clear that the larger air flow into mass spectrometer the better MS sensitivity. At the same time, the larger the air flow the larger pumps are required to maintain the good vacuum. In reality, the available pumps determine the MS air throughput. In addition, the pumping power in different vacuum sections is optimized to minimize loss of ions in transfer between sections and accommodate available pump capacities. The types of pumps used for pumping sections at different pressure can also be different. All these requirements result in typical differential pumping schemes and pressure distributions between the vacuum sections that are similar in all commercial MS systems. As an example of such similarity, the pressure in the first vacuum section in typical API designs is always about 1 Torr (practically, in the range between 0.3 to 3 Torr). Further, the 1 Torr pressure in the first vacuum section is provided by a mechanical pump, typically of a rotary vane type.
With current requirements on increasing MS sensitivity, the gas throughput in commercial MS systems has a tendency to increase because the gas throughput directly relates to the MS sensitivity. Typical solutions for increasing MS sensitivity include the use of multiple inlet capillaries and ion funnel focusing technology capable of operating at the pressures up to 30 Torr in the first vacuum section.
Portable mass spectrometry is a new direction in mass spectrometry playing an increasingly important role in numerous field applications ranging from battlefield chemical analysis to oceanographic research. As with lab instruments, the atmospheric pressure ionization methods are important for portable mass spectrometers too. However, as a small size of portable mass spectrometers automatically implies low pumping capacity, the interface to an AP ion source for portable mass spectrometers becomes a real challenge (A. Keil, N. Talaty, C. Janfelt, R. J. Noll, L. Gao, Z. Ouyang, R. G. Cooks, “Ambient Mass Spectrometry with a Handheld Mass Spectrometer at High Pressure”, Anal. Chem., 2007, vol. 79, pp. 7734-7739).
The entire contents of the references listed in this application are incorporated herein in their entirety by reference.